Efficient, high-power mechanical transducers for acoustic waves in dense media

ABSTRACT

Mechanical transducers efficiently produce and couple high-power acoustic pulses into liquid and solid media. In a transmitter, mechanical excitation of a thin transmitting plate is provided by an actuator or a motor that causes a mass to strike or a drive rod to push the thin plate. If struck, as the thin plate rings down, it delivers much of its kinetic energy to acoustic radiation in the dense medium. Different mechanisms may be used to excite the plate, and different mechanisms may be used to couple the plate excitations into dense media. Conditions are found for efficient transduction of mechanical energy by a thin plate into acoustic radiation in solid and liquid media. A receiver comprises a plate having matching resonances to the transmitting plate. Discrete narrowband frequencies of acoustic signals are used to detect phase changes in waves reflected from a moving object. By interfering successive return pulses, small changes in phase and amplitude within the reflected beam lead to large changes in interfering voltage waveforms.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present subject matter relates to acoustic transducers comprisingmechanical transmitters and matched resonant receivers and apparatusincluding the transducers.

2. Background

A conventional form of signal generation comprises an electronic signalgenerator coupled to drive a vibrating member in a manner similar tothat in which a loudspeaker is driven. An example of this form ofconstruction is illustrated in U.S. Pat. No. 7,185,447. The conventionalconstruction of this type of acoustic wave generator has signal outputpower less than input power. Use of such embodiments does not allowpulse compression, that is, production of high acoustical power withmuch lower input power.

A mechanical generator may be preferred. U.S. Pat. No. 5,274,608discloses a diaphragm in contact with water that is coupled to a mass ina cylinder for excitation. The diaphragm is vibrated by the motion of apiston coupled to the diaphragm by springs. The frequency must beregulated by means independent of the diaphragm.

U.S. Pat. No. 5,483,502 discloses what it characterizes as a high poweracoustic wave emitter. The vibrating member that produces acoustic wavesis included in a submerged housing rather than facing the water on oneside and being coupled to a source of motive power on the other side.

United States Published Patent Application Number 2012/0163126 disclosesa vibrator body comprising a composite material for generating and/orreceiving ultrasonic or acoustic waves. By forming the vibrator bodyinto a composite, the lateral mode is suppressed and the performance inthe thickness direction significantly improves. This structure providesfor a particular vibrational mode. However, mode of excitation cannot beselected once the vibrator body is configured.

Sonar transducer apparatus may also be used in a torpedo decoy. U.S.Pat. No. 4,216,534 discloses a torpedo decoy apparatus which generates arange of frequency of acoustic signals designed to attract homingtorpedoes. However, this apparatus must use preselected frequenciesrather than responding to a frequency transmitted by a torpedo.

An acoustic means of through-wall surveillance (TWS) and tracking wasdiscussed in N. C. Wild, F. S. Felber, M. Treadaway, F. Doft, D.Breuner, and S. Lutjens, “Ultrasonic through-the-wall surveillancesystem,” in Technologies for Law Enforcement, Edward M. Carapezza, Ed.,Proc. SPIE 4232, Paper 4232-29 (2000). It was found that commercialoff-the-shelf (COTS) ultrasound transducers placed against a solidbarrier produced an echo from the other side of the barrier that changedwhen someone moved behind the barrier. Then by subtracting successiveecho pulse waveforms, the difference waveform, through destructiveinterference, revealed only those persons or objects that moved betweenpulses; the echo pulse waveforms returned from stationary objectscanceled each other. The round-trip time of each pulse returned to areceiver from a moving person or object indicated its range, andtriangulating the ranges to multiple receivers indicated its location.

Acoustic TWS, and particularly handheld and portable systems, haveadvantages over alternative TWS technologies. The primary alternativesto acoustic TWS since the mid-1990s have been radar-microwave andpassive millimeter-wave sensors. Other sensor technologies, likeinfrared sensors, could image weapons concealed on a body beneathclothing, but could not image through walls. Passive millimeter-wavesensors required illumination of the targets by millimeter-waveradiation from the sky, which effectively limited their applicability tofinding persons in areas open to the sky. Radar-microwave sensors, likethe Hughes Motion Detection Radar, differential radar, radar‘flashlight,’ Time Domain's RadarVision and SoldierVision, and LivermoreNational Laboratory's Urban Eyes, were limited by attenuation in wallsto long microwave wavelengths, typically S band and longer, which didnot allow detection of the millimeter-scale motions of stationarypersons. More significantly, radar sensors could not penetrate metal ormetal-lined walls or even the aluminum-backed fiberglass insulationtypically found in homes and buildings.

SAIC's Vehicle and Cargo Inspection System (VACIS®) is designed topenetrate 15 cm of steel and image the entire contents of cargocontainers and trailer trucks, but the system elements must be bigenough to span the trucks and cargo containers it scans. Also, highlyionizing gamma radiation is not allowed for use on humans in TWSapplications, although it does image humans through thick steel wallsvery well.

The foremost advantage of acoustic TWS is that sound penetrates metalwalls almost as well as other wall materials, and does so with harmlessnon-ionizing radiation. Another significant advantage is that acousticTWS is sensitive to motions smaller than 1/10 of a wavelength. Becausethe signal processing destructively interferes successive waveforms witheach other, a movement on the scale of one wavelength will produce aninterference waveform comparable in signal strength to the successiveecho waveforms themselves. But even a movement on a scale less thanabout 1/10 of a wavelength can produce interference waveforms having agood fraction of the signal strength of the echoes.

Transducers offering advantages of high acoustic pulse power with lowelectrical input power, high efficiency, light weight, compact size, lowcost, low-voltage long-duration battery operation could be as attractivefor underwater systems as for TWS. Underwater transducers are reviewedin C. H. Sherman and J. L. Butler, Transducers and Arrays for UnderwaterSound (Springer, N.Y., 2007). Underwater transducers in a high-powerperformance range are costly primarily because of the high-voltage,high-power amplifier and power conditioning systems that are needed todrive them. Signal generation and power amplifier systems forconventional transducers involve substantial hardware. Typically, alow-level arbitrary waveform synthesizer, digitally generated and thenD/A converted, has its output amplified by a high-power amplifier andcoupled to the transducer load using a matching network. One of the moreefficient amplifiers that can reproduce arbitrary waveforms is a pulsewidth modulation (PWM) design. These power amplifier systems areavailable commercially, as are PWM amplifiers themselves.

SUMMARY

Briefly stated, in accordance with the present subject matter, there areprovided a transducer and apparatus comprising the transducer which areparticularly suited for transmitting and receiving acoustic signalsthrough dense media. A transmitter utilizes mechanical excitation of athin plate. In one embodiment, the mechanical excitation is provided byan actuator that propels a mass at the thin plate. After being struck,the thin plate then rings down, delivering much of its kinetic energy toacoustic radiation in the dense medium. Different mechanisms may be usedto excite the plate.

The receiver comprises a plate having matching resonances to thetransmitting plate. The receiving plate is the same as, or substantiallya duplicate of, the transmitting plate. Vibration of the receiving plateis sensed. Preferred sensors are piezoelectric film sensors secured tothe receiving plate with an adhesive. The piezoelectric sensors transmitreceived signals to processing circuitry.

This construction provides for high-power mechanical transducers fordense media. A mechanical impulse or mechanical pusher in a transmitterinduces an oscillation of a source to produce an oscillation of a densemedium at an interface. Oscillation may be produced by resonant impacttransmitters with matched resonant receivers, and also by tunablenonresonant direct-drive transmitters.

Mechanical transducers may utilize pulse compression. More specifically,mechanical energy is stored at low power over a long time, such as byslowly compressing a spring. The mechanical energy is released suddenlyat high power by releasing compressive forces on the spring. Forexample, less than 1 W of electrical input can produce 50 W of acousticoutput.

An important physical principle is that a properly designed thin platetransduces a mechanical impulse to acoustic energy in a dense mediumwith high efficiency at the resonant mode frequencies of the plate.

Apparatus comprising the transducer may comprise, for example, athrough-wall sensor or a sonobuoy that can autonomously performlong-duration underwater surveillance. Apparatus for other applicationsrequiring transmission or reception of acoustic waves may also beprovided.

There is a need to couple efficiently the acoustic energy produced by atransmitter into the dense media of water or solid walls, for example.Designs of transmitter plates and efficient means for transmitters tocouple their kinetic energy of oscillations into acoustic powertransmitted into dense media are described.

In implementing the present subject matter, a tunable mechanicaltransmitter was developed that, operating on a single 9-V battery, wascapable of transmitting more acoustic power into a wall than a heavierCOTS transmitter operating at 1800 volts peak-to-peak. Besidestunability, therefore, a significant advantage of this mechanicaltransmitter was safety. In another implementation, a narrowbandmechanical transmitter, driven by a 35-W motor, was able to detectmotion of a person through two thick walls and over 6 meters, evenwithout the additional 26-dB of signal-to-noise improvement that couldhave been achieved in signal processing with a narrowbandfilter/amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

The present subject matter may be further understood by reference to thefollowing description taken in connection with the following drawings:

FIG. 1 is an illustration of a deployed sonobuoy comprising a transducerapparatus;

FIG. 2 is an illustration of a transducer apparatus used as athrough-wall detector;

FIG. 3 is a block diagram of a transducer apparatus coupled to a medium;

FIG. 4 is a side view of a transducer apparatus illustrating atransmitter;

FIG. 5 is a top view and side view of a transducer apparatusillustrating a receiver;

FIG. 6 is an elevation of one embodiment of a plate driver;

FIG. 7, consisting of FIGS. 7A, 7B, 7C, and 7D, illustrates differentembodiments of plate drivers;

FIG. 8 consists of FIG. 8A and FIG. 8B, which are illustrations of afurther implementation of the plate driver and a tunable mechanicaltransmitter respectively;

FIG. 9 is a view of a transmitting plate;

FIG. 10 illustrates a means for selecting positions of piezoelectricsensors on a plate;

FIG. 11 is a graph illustrating efficiency of energy transfer forelastic impacts of impactor mass m on plate of effective mass M_(e);

FIG. 12 is an illustration of model for transmission of sound from atransmitting plate into wall across air gap;

FIG. 13 is an illustration of sensors mounted in series on a receivingplate;

FIG. 14 illustrates sensors mounted for operation in a dipole mode;

FIG. 15 is a waveform chart illustrating radiated power vs. time (solidcurve) for point design of 1-kHz undersea impact transducer; averagedover wave cycle (dashed);

FIG. 16 is a block diagram of a module for processing received audiowaves;

FIG. 17 is a functional block diagram of a computer program forprocessing the received audio waves;

FIG. 18 is a waveform chart illustrating operation of an unfilteredreceiver in a through-wall monitoring system;

FIG. 19 is a waveform chart illustrating operation of a filteredreceiver in a through-wall monitoring system;

FIG. 20 is a waveform chart illustrating a detection display of presenceor absence of a person on an opposite side of a wall;

FIG. 21 is a functional block diagram of a computer program forprocessing the received audio waves for locating and tracking persons onan opposite side of a wall; and,

FIG. 22 is an illustration of one embodiment of a variable-depth FASNunit.

DETAILED DESCRIPTION

The present subject matter comprises mechanical transducers that areparticularly well suited for efficiently and inexpensively producing andcoupling high-power acoustic pulses into dense media, like walls andwater. Advantages of an acoustic TWS sensor and its signal processinginclude high-resolution locating and tracking, portability, low cost,quick and easy preparation and deployment, near-real-time dataprocessing and display, and no damage or changes to the wall.

One significant application of the present subject matter is inthrough-wall surveillance (TWS). Advantages of acoustic TWS over radarTWS include detection through metal and metal-lined walls and detectionof stationary persons through walls.

The present subject matter overcomes the shortcomings of prior TWSsensors that can detect only moving persons, giving a user falseconfidence that all persons on the other side of a wall are accountedfor. This can make a life-or-death difference in military or lawenforcement operations.

The present subject matter uses discrete narrowband frequencies todetect phase changes in waves reflected from a moving person. Byinterfering successive return pulses, small changes in phase andamplitude within the reflected beam lead to large changes in receivedvoltage waveforms. This allows detection of mm-scale motion by cm-scalewavelengths. It has been found that detecting the mm-scale motion ofpersons who are breathing but otherwise stationary should be performedusing narrowband frequencies.

Another significant application of the present subject matter is inlong-duration surveillance by sonobuoys. Underwater impact transducersare particularly well suited for undersea surveillance by a networkedsystem of fixed autonomous sonobuoy nodes (FASNs) near harbors, bays,and sensitive shoreline installations.

The present subject matter comprises a transducer apparatus particularlysuited for providing low-cost, high-power acoustic wave generators andreceivers for processing sound waves coupled through dense media. Whilethe present subject matter is not so limited, it provides particularadvantages in the context of high-density media. Examples ofhigh-density media include seawater and steel walls.

Many different applications may be implemented through use of thepresent subject matter. Particularly useful applications includesonobuoys providing long-duration surveillance in and near harbors andother littoral waters. Another application is sensing of moving objectsthrough the wall of a cargo container or the like. Two exemplaryapplications are illustrated. However, many other applications areavailable.

The Figures describe a variety of devices. FIG. 1 is an illustration ofthe transducer apparatus 1 comprising a sonobuoy 20. The sonobuoy 20 isdisposed in a medium 10 such as a body of water 12 having a watersurface 14 and a bottom 16. The sonobuoy 20 includes a buoy 24 whichfloats on the water surface 14 or at some height above the bottom 16 andis anchored to an anchor 36 resting on the bottom 16. A tether 18secures the sonobuoy 20 to the anchor 36. The sonobuoy 20 includes acommunications antenna 26. A sonar head 28 is suspended in the water 12beneath the buoy 24. The sonar head 28 is electrically coupled to theantenna 26 and mechanically supported to the buoy 24 at a fixed distancebelow the buoy 24. A tension reel 32 attached to the tether 18continually adjusts the length of the tether 18 to accommodate themotion of the water 12. Consequently, the buoy 24 remains substantiallyin horizontal registration with the position of the anchor 36. Furtherdetails of the sonobuoy 20, its components, and operation are discussedbelow.

FIG. 2 is an illustration of the transducer apparatus 1 comprising athrough-wall detector apparatus 50. The detector apparatus 50 comprisesa transducer head 60 coupled to a power supply, propulsion, andcommunications and processing module 64. In operation the transducerhead 60 is positioned at a wall 72 of a container 70 by a support 66.The wall 72 in a nominal application is a steel wall 72. The container70 may be mounted on a truck 76 and may comprise a cargo container orthe cargo area of the truck 76.

FIGS. 3, 4, and 5 are discussed together. The same reference numeralsdenote the same components. FIG. 3 is a block diagram of the transducerapparatus 1 illustrating one embodiment of a concept. FIG. 4 is a sideview of the transducer apparatus 1 illustrating the transmitter 102.FIG. 5 is a top view and side view of the transducer apparatus 1illustrating the receiver 104.

FIG. 3 is shown as a discrete unit solely for the purposes of thepresent discussion. In selected preferred embodiments, the transmitterand receiver need not be in the same enclosure. It is not essential forthe transmitter and receiver to be collocated.

A housing 100 is provided. In the present illustration, a housing 100comprises a rectangular parallelepiped. However, it is not essential forthe housing 100 to have any particular shape. It is not essential thatthe housing 100 include one enclosure. Where the housing 100 comprisesmore than one enclosure, the enclosures may be placed at separatelocations. A transmitter 102 and a receiver 104 are mounted in thehousing 100. The transmitter 102 comprises a plate driver 154 and atransmitting plate 120, which produces acoustic waves to be propagatedinto the medium 10. The transmitter 102 also comprises a means 112 ofcoupling the plate 120 to the medium 10, and a means of causing theplate to vibrate, further described below. Because the normal-modefrequencies of a plate of a given size and shape are roughlyproportional to the sound speed in the plate, and the weight isproportional to the density, the plate 120 is preferably comprised of amaterial with a high sound speed and a low density. The materialstrengths and hardness are preferably high to withstand many impacts.Aluminum and titanium are preferred materials. Copper, steel, or nickelcould be used.

The transmitting coupler 112 couples acoustic waves 114 to the medium10. In transmitting to a solid surface, such as the wall 72 in FIG. 2,the transmitting coupler 112 may comprise spacers to maintain a thin gapbetween the transmitting plate 120 and the wall 72. A thin gap ispreferred. In the embodiment of FIG. 2, the transmitting coupler 112effectively utilizes the wall 72 as a sound board. In the embodiment ofFIG. 1, the transmitting coupler 112 may comprise a surface of thetransmitting plate 120. More than one transmitting plate 120 may beprovided.

Each transmitting plate 120 has a transmitting front surface 122 and atransmitting rear surface 124. The terms “front” and “rear” arearbitrary. The transmitting front surface 122 faces the medium 10. Thetransmitting rear surface 124 faces the plate driver 154.

Received waves 126 are coupled from the medium 10 by a receiving coupler128. In the embodiment of FIG. 2, the receiving coupler 128 may comprisea solid bridging material that indirectly transfers acoustic vibrationsfrom the medium 10 to the receiving plate 130. In the embodiment of FIG.1, the receiving coupler 128 may comprise a surface of the receivingplate 130.

Each receiving plate 130 has a front receiving surface 132 and a rearreceiving surface 134. The terms “front” and “rear” are arbitrary. Thefront receiving surface 132 faces the medium 10. The rear receivingsurface 134 has receiver sensors 136 monitoring the surface as by laservibrometers that are not affixed. The receiver sensors 136 arepreferably piezoelectric film sensors adhesively affixed to the rearreceiving surface 134.

A selectable number of receiver sensors 136 respond to oscillations ineach receiving plate 130 to provide output signals indicative of thereceived waves 126. The output signals from the receiver sensors 136 arecoupled to a signal processor 140.

Within the transducer head or heads 60 (FIG. 2) the receiving plate 130differs from the transmitting plate 120 in that it does not need to beattached to a more massive structure like the transmitter. Thetransmitting plate 120 should be acoustically isolated from the housingor housings, 100 and the transmitter 102 structure in the housing 100.In this manner, resonances of the transmitter structure do not couple toand affect the resonances of the transmitting plate 120. This isolationmay be accomplished by using rubber acoustic isolators 133. Both thetransmitting plate 120 and the receiving plate 130 may be supported by arigid support frame 131.

FIG. 6 is the elevation of a dual-plate impact-transmitter designincluding a first transmitting plate 120 and a second transmitting plate121. The first and second transmitting plates 120 and 121 are mounted infirst and second frames 168 and 169 respectively. In this embodiment,the plates 120 and 121 combine to produce a unique double-pulsesignature while each provides signals at equal or substantially equalfrequencies, bandwidths, and pulse widths. Impact transmitters have asharply defined pulse start time, t=0. Piezoelectric film sensors 137affixed to the transmitting plates 120 and 121 register a sharplydefined voltage spike at the moment t=0 of impact of the impactor 166 onthe first transmitting plate 120 and at the moment of impact of theimpactor 166 on the second transmitting plate 121. By this method, thejitter Δt in determining the moment of impact on each plate is very muchsmaller than a wave period, and generally could be no more than onevoltage sampling time. The high precision to which the moment of impacton each plate is known can be used in the following way to assign aunique pulse signature to each dual-plate fixed autonomous sonobuoy node(FASN) sonar head after it has been manufactured.

A prime mover 160 produces motion of the impactor 166 mounted on a stalk164. The prime mover 160 causes the impactor 166 to strike the firstplate 120. The prime mover 160 then causes the impactor 166 to strikethe second plate 121. The offset distance of the impactor 166 to thesecond plate 121, shown as L_(i) in FIG. 6, is adjustable. Either theposition of the second plate can be adjusted by means of a sliding mount170, or the impactor position between two fixed plates can be adjusted.When L_(i) is changed, the difference in the pulse start times of thetwo plates 120 and 121 is changed. Since the pulse start time at eachplate is known to high precision, the difference in the times is alsoknown precisely and can be used to identify the source in a network ofmany such FASNs.

Each thin plate 120 and 121 could have a button, called an anvil 174,affixed to it. The anvil 174, which is preferably steel, has severalpurposes. It protects the thin plate from potentially hundreds ofthousands of impacts of the impactor 166 to be expected during thelifetime of the plate. The impactor 166 and stalk 164 together arereferred to as the ‘hammer’. A substantially elastic collision of theanvil 174 with the impactor 166, which is also preferably steel, isprovided. The anvil may help to put more of the impactor kinetic energyinto exciting the desired mode of oscillation and less into parasiticmodes, if the hammer strikes slightly off the intended strike point.

In one illustration for the baseline design of a network of hundreds of1-kHz FASNs, the sampling rate of the receiver signal processing is 32samples per ms. The difference in start times of the pulses from each ofthe two faces, therefore, may be known to within about 30 μs. The offsetL_(i) is adjustable over about 1 cm. The velocity of the impactor 166upon impact is of the order of 1 m/s. That means the difference in pulsestart times at the two plates 120 and 121 can vary over about 10 ms asL_(i) is adjusted over the 1-cm adjustable range. In this example, ofthe order of 300 uniquely distinguishable offset positions of the secondplate are available over the 1-cm adjustable range. The adjustments onthe FASNs can be made either by dialing in random offsets L_(i) or byfine positioning by means of a micro-positioning knob, similar to amicrometer, controlling a worm-screw drive.

Upon producing a double pulse, a FASN may transmit to either a relaybuoy or to a central station a signal conveying its location and thetime difference between the start of its two pulses. Any FASN receiverthat receives this double pulse and then a short time later receives adouble pulse reflected off a target, such as a submarine, could thentransmit a signal conveying its own location and the times of receivingthe pairs of double pulses. From this information, the range of thetarget to the receiver may be determined. If at least two FASNs in thenetwork report a detection, then the position of the target can bedetermined by triangulation.

This method of identifying the source depends upon both pulses from asingle FASN reflecting from a target and being received at any otherFASN in the network. That means the method depends upon the impacttransmitters being essentially simple sources with nearly isotropicradiation patterns from each plate.

The transmitting plate 120 is struck by the impactor 166. In order tofacilitate interaction between the transmitting plate 120 and theimpactor 166, the anvil 174 is placed on the rear surface 124 of thefirst transmitting plate 120 and on the rear surface 124 of the secondtransmitting plate 121. The anvil 174 may be affixed to the point ofimpact 180 on the transmitting plate 120 to protect, regularize, andenhance the elasticity of the point of impact 180. The points of impact180 and the anvils 174 may be positioned at various locations on thetransmitting plates 120 and 121 depending on the modes of vibration tobe induced, as further described with respect to FIG. 9 below.

FIG. 7, consisting of FIGS. 7A, 7B, 7C, and 7D, illustrates differentembodiments of plate drivers 154. Each of FIGS. 7A, 7B, 7C, and 7D is amechanical schematic elevation. FIG. 7A corresponds to a conventionalembodiment of a loudspeaker driven by a linear induction motor 160 b.FIG. 7B corresponds to the embodiment of an actuator 160 a illustratedin FIGS. 4 and 8 a. FIG. 7C could correspond to the embodiment of anoscillator motor 160 c driving the impact transmitter illustrated inFIG. 6. FIG. 7D could correspond to the embodiment of the actuator 160 adriving the impact transmitter illustrated in FIG. 6. The prime mover160 in FIG. 6 could comprise an oscillating actuator 160 c, as in FIG.7C, or a leveraged actuator, as in FIG. 7D. In the embodiments of FIGS.7A, 7B, 7C, and 7D the prime mover 160 could comprise anelectromechanical or electromagnetic actuator or linear induction motor.

In FIG. 7A, the prime mover 160, linear induction motor 160 b orotherwise, is directly coupled to the transmitting plate 120 to providetunable direct drive, in a manner similar to a loudspeaker drive. Thisembodiment can produce complex acoustic waveforms having characteristicsof voice or music. The principal disadvantage of direct drive in thiscontext is that the displacement of the oscillating surface of the platelimited to the stroke of the motor. However, direct drive of the thinplate 120 by the linear induction motor 160 b at a variable position onthe plate 120 that preferentially excites a desired mode of oscillationat a desired frequency can be an efficient means of producing ‘tunable’narrow-band pulse radiation, particularly under water or in other liquidmedia. For example, if the linear induction motor 160 b drives the plate120 at the resonant frequency of the dipole mode of the plate 120 and ata radial position on the plate at which the dipole mode amplitude isgreatest, the motor 160 b can cause the dipole oscillation amplitude togrow exponentially, and the radiated power at the dipole mode frequencyto grow exponentially in power, subject to the limitations of thequality factor of the dipole mode. By reciprocation in the linearinduction motor 160 b, the same direct-drive mechanism that produces thetunable but resonant narrow-band radiation can be used to receive thereflected pulse radiation.

As seen in FIG. 7B, the actuator 160 a moves the impactor 166 in areciprocating, linear motion to periodically strike the transmittingplate 120 and recoils to permit vibration of the transmitting plate 120.In the embodiment of FIG. 7D, the impactor 166 is pivotally mounted to abase 150. A drive rod 176 coupled to the stalk 164 is moved in areciprocating, linear motion to cause the impactor 166 to strike thetransmitting plate 120 and then to remove the impactor 166 fromengagement with the transmitting plate 120.

FIG. 8A is a diagram illustrating a further implementation of the platedriver 154. In this embodiment, the prime mover 160 comprises anelectric motor 190 powered by a battery or batteries 192. The electricmotor 190 drives a worm-screw drive 194. The worm-screw drive 194 iscoupled to linearly displace a latch 196. A spring 200 is mounted in abarrel 202 anchored at an end which is remote from the transmittingplate 120. An opposite end of the spring 200 supports the impactor 166.The impactor 166 is preferentially spherical. Because a hard, durable,high-density impactor material with a high sound speed is preferred,steel is a preferred impactor material. The worm-screw drive 194 rotatesto displace the latch 196 to compress the spring 200. At a preselectedpoint and time, the latch 196 releases the spring 200, and the impactor166 is propelled by the spring 200, causing the impactor 166 to impactthe transmitting plate 120.

FIG. 8B is an illustration of a tunable mechanical transmitter 205. Athin plate (flat surface) 206 is caused to vibrate by plucking a wire208 attached under tension provided by a tensioner 215 to the plate 206,rather than by high voltages. The wire 208 is plucked by abattery-operated mechanical means 207, such as a motor. In theembodiment of FIG. 8B, the transmitter 205 comprises the thin plate 206,the wire 208 under tension, a wire plucking means 209, and a supportingstructure 210. The vibrating surface, the plate 206, in this particularillustration is a thin, circular aluminum plate, held tightly against alip 211 on the bottom of the structure 210 by the tension of the wire.The tensioned wire 208 may be perpendicular to the plate 206 and may beattached to the center of the plate by a small block 212 cemented to theplate 206. The wire 208 may be held in the block 206 by opposingsetscrews 213. When the wire 208 is plucked, it causes the plate 206 tooscillate longitudinally at twice the frequency of transverse waveoscillations on the wire 208, which may in general be much higher thanthe natural frequency of the plate 206. The thin plate 206 effectivelyacts as a spring, with the stiffness of the plate providing therestoring force.

The mechanical means of plucking the wire 208 is a rotating sawtoothwheel 214. Each tooth 209 of the wheel 214 is shark-fin shaped, with aconvex gently curved edge and an acute straight edge. The wire 208 ridesup on the curved edge and “falls” over the straight edge. In this way,the wire falling over the tooth 209 edge initiates discrete wavetrainpulses. The pulse repetition frequency is determined by the toothspacing and by the rim speed of the wheel 214.

The mechanical transmitter 205 has a tunable frequency by adjusting thetension in the wire 208 by means of the tensioner 215. In one examplethe oscillating displacement of the plate 206 at the driving frequencyof the wire 208 is only about 0.15 microns, or about 0.005 times theinitial displacement of the wire 208 at the point where it was plucked.Since power is proportional to the square of the plate displacement, anontunable, resonant configuration has orders of magnitude greaterpower.

This structure provides an efficient coupling of acoustic energy tosolid walls. This structure allows for production and coupling ofhigh-power acoustic pulses to all liquid and solid media. This structureis well-suited to use in a sonobuoy surveillance network. Advantages ofthe mechanical transducers herein include high acoustic pulse poweroutput, low electrical power input, high efficiency, light weight,compact size, low cost, and low-voltage long-duration battery operation.These advantages are available in underwater systems and TWS systems.

FIG. 9 is a view of one form of transmitting plate 120. The transmittingplate includes an outer axial surface 220. In one preferred form, thetransmitting plate 120 is circular and is secured by the frame 168 (FIG.6) around a circumference 222. In the present context, the circumference222 includes a portion extending radially inwardly of the outer axialsurface 220 that is held by the frame 168. The anvil 174 is placed onthe rear surface 124.

In one embodiment, the transmitting plate 120 transmits acousticradiation into water at a frequency of 1 kHz and at an average power of50 W. Considerations for selection of transmitting parameters andtransmitting plate 120 specifications are discussed below. An exemplaryset of specifications for an impact transmitter point design of anedge-clamped circular aluminum plate that can radiate 50-W, 4-ms pulsesinto sea water at 1 kHz is set forth in Table I.

TABLE I Parameter Value Plate radius, cm b = 14 Plate thickness, cm d =1.3 Initial specific energy, J/kg E₀ = 0.21 Plate frequency in air, kHzf_(p) = 1.6 Plate area, cm² A = 650 Active area, cm² A_(a) = 120 Activeradius, cm a = 6.1 Plate mass, kg M = 2.2 Active mass, kg M_(a) = 0.41Reactive mass, kg M_(r) = 0.63 Effective mass, kg M_(e) = 1.0 Mass ratioμ² = 2.5 Initial effective energy, J M_(e)E₀ = 0.22 Angular frequency,kHz ω = 6.3 Wavenumber, cm⁻¹ k = 0.042 Diffraction constant ka = 0.26Spring constant, kg/(ms)² K = 41 Max. displacement, cm z₀ = 0.010 Max.velocity, cm/s u₀ = 65 Radiation resistance, kg/ms R_(r) = 0.60Radiation reactance, kg/ms X_(r) = 3.9 Peak radiated power, W P₀ = 260Average radiated power, W P_(r) = 50 Decay rate, kHz Γ = 0.58 Bandwidth,kHz B = 0.091 Quality factor Q = 11

Alternatives to the central point of impact 180 of FIG. 6 areillustrated in FIG. 9. In FIG. 9, a selectable point of impact 230 isprovided. The vibration mode which is produced correlates with theselectable point of impact 230 on which the transmitting plate 120 isstruck. In one preferred form, the circular transmitting plate 120 isstruck at a center 232 on the anvil 174, which excites primarily thefundamental mode of vibration of the transmitting plate 120. Setting theselectable point of impact 230 at other positions, such as 236 or 238,will excite different combinations of modes of vibration with differentfrequencies.

In general, a multiplicity of the receiver sensors 136 are preferablyattached to the rear surface 134 of the receiving plate 130, as shown inFIG. 10. The receiver sensors 136, which are piezoelectric film sensorsin this embodiment, are attached with their polarities, corresponding tothe vibrational mode that is being measured, in series. If n sensorsattached in series each measure the same voltage response with the samepolarity, the signal strength will be amplified by n², because signalstrength scales as voltage squared. For example, the receiving plate 130in FIG. 10 has four piezoelectric film receiver sensors 136 mounted onits rear side 134 in a symmetrical square array around the center 232 ofthe receiver plate 130. Flexible sensor leads 138 were soldered to eachother in series at a 10-pin header enhancing the signal voltage receivedfrom fundamental-mode oscillations of the plate 130 by a factor of about4 over a single sensor and the signal strength by a factor of about 16.

As illustrated in FIG. 14, which is described below, for somevibrational modes, such as dipole modes, one piezoelectric film receiversensor 136 may be stretched over a convex bend 260 while at the samemoment another is compressed over a concave bend 262. In that case,their voltage polarities will be opposite each other, and their leadsshould be connected positive to positive or negative to negative inorder for their voltage signals to add constructively when monitoringthat particular mode of vibration.

If the receiving plate 130 is not the same plate as the transmittingplate 120 used for transmitting pulses, in a preferred example, it wouldbe identical, and the mounting, bonding, and fastening of the receivingplate 130 would be identical to the mounting, bonding, and fastening ofthe transmitting plate 120. The plates 120 and 130 and mountings aresufficiently alike if the normal mode resonant frequencies of bothplates will be within a tolerance providing a degree of accuracy suchthat the bandwidths of the useful resonances at the receiversubstantially overlap the corresponding bandwidths at the transmitter.It may be preferable to use a strong bonding agent, in addition tomechanical fasteners such as bolts, to provide uniform edge-clampedboundary conditions in both plates. Uniformity in edge-clamped boundaryconditions also enhances the quality factor Q of both plates.

Acoustic waves which strike each receiving plate 130 must be sensed. Onepreferred way to sense the acoustic waves is to affix the receiversensors 136 to the receiving rear surface 134 of the receiving plate130. The piezoelectric receiver sensor 136 responds to vibrations.Vibrations are sensed at areas of stress in the receiving resonant plate130. FIG. 10 illustrates the concept of placing receiver sensors 136 atlocations on the vibrating receiving plate 130 where stresses aregreatest because curvature of the plate 130 is greatest for theparticular vibrational mode that is being measured.

FIG. 10 is also a rear view of the receiving plate 130 having at leastone receiver sensor 136 mounted on the rear receiving surface 134. Onepreferred form of receiver sensor 136 is a piezoelectric film. Amultiplicity of receiver sensors 136, 136-1 through 136-n, where n is aninteger, may be provided. Receiver sensors 136 may each be coupled to apair of respective sensor leads 138. In an embodiment of FIG. 10, thepiezoelectric receiver sensors 136 are mounted on a 10-cm diameter,1-mm-thick aluminum plate at a location on the plate where it would bemost responsive to the flexing of the fundamental mode. An effective wayto optimize the positioning of the piezoelectric receiver sensors 136 isby repeatedly measuring the in-band response of a sensor to a calibratedsignal as the sensor is repeatedly lifted and adhesively fastened ontonew positions.

FIG. 11 is a graph illustrating efficiency of energy transfer forelastic impacts of impactor mass m on a plate of effective mass M_(e).The effective mass M_(e) is defined as the sum of the active mass M_(a)of the transmitting plate and the effective reactive mass M_(r) of themedium in front of the plate (see Table I). If impact duration, thecontact time of the impactor 166 on the transmitting plate 120, is toolong for efficient transfer of kinetic energy from the impactor 166 tothe plate 120, then the impactor 166 must deliver the necessary energyto the plate 120 at higher velocity and lower mass and lower efficiency.The energy efficiency of elastic impacts on a plate falls with impactormass as ε=4(m/M_(e))/(1+m/M_(e))² for m less than M_(e), as shown inFIG. 11.

FIG. 12 is an illustration of a model for transmission of sound from atransmitting plate 120 into wall 72 across an air gap. The intensitytransmission coefficient from plate to wall scales with air gap L asT_(I) L⁻². The difference between an air gap of 1 mm and 0.2 mm is 14 dBin transmission into the wall 72. The air gap L should not be made sothin, though, that the maximum amplitude z₀ of the plate vibrationscauses the plate to contact the wall. That is, transmission into thewall is maximized for the narrowest gap subject to L>z₀.

FIG. 13 is an illustration of a series of sensors mounted on thereceiving plate 130. In general, a multiplicity of sensors can andshould monitor vibrations on the receiving plate 130. The sensors shouldhave their polarities, corresponding to the vibrational mode that isbeing measured, connected in series. If n sensors attached in serieseach measure the same voltage response with the same polarity, thesignal strength will be amplified by n². For example, the receivingplate 130 in FIG. 13 has four piezoelectric receiver sensors 136-1through 136-4 mounted on its receiving rear surface 134 in a symmetricalsquare array around the center 232 of the plate. This configurationenhances the signal voltage produced by the fundamental mode ofvibration by a factor of 4 over a single sensor and the signal strengthby about 12 dB, a factor of 16.

FIG. 14 illustrates sensors mounted on a partial, enlarged side view ofthe receiving plate 130 for operation in a dipole mode. The scale isgreatly exaggerated in the horizontal direction relative to the verticaldirection. One piezoelectric receiver sensor 136-1 may be stretched overthe convex bend 260 while another receiver sensor 136-2 is compressedover the concave bend 262. In that case, their voltage polarities willbe opposite each other, and their leads 138 should be connected positiveto positive or negative to negative in order for their voltage signalsto add constructively.

FIG. 15 is a chart that illustrates a typical exponentially decayingpattern of acoustic power radiated into a medium by an impacttransmitter. Radiated power is shown in the solid curve for the pointdesign of the 1-kHz undersea impact transducer given by Table I. Thepower averaged over a wave cycle is illustrated by the dashed line. Inthis example, the average power radiated during 4 ms is 50 W. In thisexample, the average electrical input power to produce a pulse every 5seconds is 0.7 W.

A highly effective and efficient way to couple acoustic energy into thedense medium 10, such as a wall or water is provided. A properlydesigned thin plate transduces a mechanical impulse to acoustic energyin a dense medium with high efficiency at the resonant mode frequenciesof the plate.

Acoustic energy is transmitted across an interface into a medium only byproducing a mechanical motion of the medium within a wavelength of theinterface. And an effective and efficient way to produce a mechanicalmotion of the medium is by inducing a purely mechanical motion of theinterface, such as by a mechanical pusher or by a mechanical impulse. Ifthe interface is moved by purely mechanical means, then the onlysignificant energy losses, other than in producing the mechanicaldriving motion, are in the mechanical linkages or in the inelasticity ormomentum-transfer mismatch of the impacts, which can be made small.

This power transmission uses the excitation of a thin plate as shown inFIG. 6. An actuator propels a mass, called the impactor 166, at the thinplate 120. The mass impacts the plate and bounces back, exciting thefundamental mode if striking the center of a symmetrical plate, andexciting higher frequency modes if striking off center. The thin plate120 then rings down, delivering much of its kinetic energy to acousticradiation in the medium 10, if coupled properly.

For coupling a transmitter 102 (FIG. 16) to water or other liquid media,in one preferred embodiment, the transmitting front surface 122 drivesthe medium 10 directly. For an impact transmitter in a liquid mediumsuch as the body of water 12 (FIG. 1), after the impactor 166 hasdelivered a momentum impulse and kinetic energy to the transmittingplate 120, the primary avenue for that kinetic energy to be spent is bydoing acoustic work on the medium 10.

The maximum kinetic energy of a circular edge-clamped plate of mass M iscalculated as a function of the maximum plate velocity at the center u₀,and is found to be 0.182(Mu₀ ²/2), which is equal to the maximum kineticenergy of a flat circular piston having the same velocity, u₀, but amass of only 0.182M. Because only the central region of the edge-clampedplate moves much, the active plate mass M_(a) is only 18.2% of theactual mass. That means the active area of the plate is A_(a)=0.182A,and the active radius is a=0.427b, where b is the actual radius of theplate. Different boundary conditions would result in different activemasses and areas.

When the impact transmitter is submerged in water or other dense fluid,the fluid increases the effective mass of the plate, decreases theresonant frequency, increases the maximum acoustic power output, andbroadens the bandwidth, compared to the performance of the sametransmitter in air.

When operated under water, the mode frequencies of a thin plate aredownshifted. The effect of the water is to increase the effective massof the plate. For a given spring constant, a greater mass oscillates ata lower frequency. If the spring constant of the plate is stiff, thenthe water can be inconsequential. For example, the oscillations of aheavy-duty spring from the strut on a car would be virtually unaffectedby water. But if the spring constant of the plate is weak, then thewater can significantly downshift the resonant mode frequencies.

FIG. 16 is a block diagram of steps for processing acoustic reflectionsat receivers into detections of persons on the other side of a wall orinto an x-y map giving their real-time locations. In one embodiment, thetransmitter 102 produces periodic periodic acoustic pulses. Each strikeof the impactor 166 on the transmitter plate 120 produces acoustic waves114 in the medium 10. Each strike of the impactor 166 on the transmitterplate 120 also triggers the receiver 104, preferably by producing asharp voltage spike in a piezoelectric film sensor 137 attached to thetransmitting plate 120 to serve as a trigger.

In the receiver 104 receiver sensors 136 sense received acoustic waves126. The receiver sensors 136 convert these waves into electricalsignals. An analog-to-digital (A/D) converter 330 converts the signalsto digital data for processing as described with respect to FIG. 17.

Steps in FIG. 16 are explained more fully below with respect to FIG. 17.An example of through-wall detection made by the impact transducer 1 isprovided.

The signal processing for each transmitted pulse begins with theimpactor 166 striking the transmitting plate 120. That event triggersthe start of a delay period of some number of milliseconds. For example,to trigger the delay period, the impactor can produce a large voltagespike in a trigger unit 308 from the piezoelectric film sensor 137affixed to the transmitting plate 120. The delay period is needed sothat the sound pulse produced by the impact and conducted through theair and through the walls to the receivers will be sufficientlydiminished at the receivers by the end of the delay period. Thedisadvantage of the delay period is that persons in the immediatevicinity of the transmitter will not be detected. The longer the delayperiod, the greater the ‘dead zone’ for detection.

After the delay period ends, the signal processor begins acquiringvoltage waveforms from the receiver sensors 136. If it is only requiredto detect humans, and not to locate or track them, then only onereceiver may be necessary. Otherwise, a horizontal linear array of atleast two receivers is needed. The voltage waveforms from each receiverare then digitized by the A/D converter 330.

During the delay period an output such as a square wave 404 of zeroamplitude may be used to inhibit processing of received signals. In thepresent illustration the A/D converter 330 provides digitized input datafor processing. The trigger unit 308 is programmed to turn the A/D to an“off” state for the duration of the square wave 404.

The digitized waveforms carry information about the acoustic wavesreflected from all objects and persons on the other side of a wall.Since the receivers may also receive reflections from the same side ofthe wall, it may be necessary to operate the system remotely, asillustrated by the embodiment in FIG. 2, if transmitter back lobes andside lobes cannot be suppressed. The next steps in signal processing areto remove reflections from completely motionless objects.

At all times, a running average is kept in a first memory 350 of thepast n waveforms from each receiver. Since the noise level will besuppressed and S/N will be increased by a factor of about n^(1/2), thenumber of waveforms in the average should be as large as allowed byoperational constraints. The newest waveform can be set to start atexactly the same time delay with respect to the impactor 166 strikingthe plate 120 as the average waveform. Or at least the difference intime delays with the average waveform will be very much less than thewave period. If not, then it may be necessary to adjust the start of thenewest waveform to correspond to the start of the average waveform. Thiscan be done by cross-correlating the newest waveform with the averagewaveform in the first memory 350 using a correlator 360.

After t=0 has been established for the newest voltage waveform for eachreceiver at the correlator 360, a bandpass filter 364 is applied to thewaveforms. The filter 364 should be matched to the resonant frequency orfrequencies of the transmitter and receiver. The bandwidth or bandwidthsof the filter 364 should be matched to be no less than about the fullwidth at half maximum (FWHM) of the spectral resonances. Generally, aslong as the bandwidth encompasses the FWHM, the narrower the bandpass ofthe filter 364, the more noise is excluded from the signal and thehigher the signal-to-noise ratio, S/N. But the bandpass filter 364should not be so narrow that it excludes a significant part of thein-band resonant energy, or that it reduces probability of detection.

After the waveforms have been filtered, the filtered waveform from eachreceiver is compared to, and subtracted from, a running average of thepast n filtered waveforms from that receiver. The running average isprovided from a second memory 366. The newest filtered waveform isdestructively interfered with the most recent average filtered waveformin the second memory 366 at a difference-waveform generator 370.

The output of that waveform interference process, the differencewaveform, should only differ significantly from noise for thosereflections from targets that are not motionless. The differencewaveform is amplified with an automatic gain control (AGC) 380. The AGCis designed to compensate for the weaker reflections arriving from moredistant targets. The power of the diffusely reflected sound wavesreceived from a target scales with range r as approximately r^(−3.8).The time-of-flight of the reflected sound waves is measured, and therange is roughly half the time-of-flight times the sound speed in air.

The filtered and amplified difference waveforms from one receiver may besufficient to detect persons through a wall. To locate and track personsrequires at least two receivers. The difference waveforms may beoperated on by detection or tracking algorithms 390. If the transmit andreceive beam divergence is narrow relative to the area to be scanned,then a linear array of more than two receivers and/or transducers may beneeded to cover fully the area to be scanned. In that case differencewaveforms can be created from neighboring pairs of receivers, or withappropriate modifications to tracking algorithms, difference waveformscan be created from more than two waveforms, with some improvement inresolution, particularly cross-range resolution.

FIG. 17 is a functional block diagram of a computer program forprocessing received audio waves 126 in a through-wall embodiment. FIG.17 is also a summary of certain operations described with respect toFIG. 16. At block 400, a processing cycle for each transmitted pulsebegins with a trigger signal being generated. At block 402, productionof a delaying pulse is commanded. At block 404, a delaying pulse isgenerated. In one preferred embodiment, the delay is on the order ofmilliseconds. During the delay period received audio waves 126 are notprocessed. The delay period is needed so that the noise produced by theimpact and conducted through the air and through the wall to thereceivers will be sufficiently diminished at the receiver by the end ofthe delay period. At block 406 it is sensed whether the delay hasfinished. If so, operation proceeds to block 408. If the delay is notfinished operation returns to the input of block 406.

At block 408 signal processing begins. A running average 408 is kept ofwaveform data in the first memory 350. The signal processor beginsacquiring voltage waveforms from the sensors on the receivers. Thevoltage waveforms from each receiver are then digitized in the A/Dconverter 330. To detect humans, at least one receiver is needed. Tolocate or track them, an array of at least two receivers is needed.

The digitized waveforms carry information about the acoustic wavesreflected from all objects and persons on the other side of the wall.Since the receivers may also receive reflections from the same side ofthe wall, it may be necessary to operate the system remotely iftransmitter back lobes and side lobes cannot be suppressed. The nextsteps in signal processing are to remove reflections from completelymotionless objects.

A preselected number n of waveforms from each receiving module memory isused to produce a running average. Since the noise level will besuppressed and S/N will be increased by a factor of about n^(1/2), thenumber of waveforms in the average should be as large as allowed byoperational constraints.

At block 410, signal conditioning is performed. For example, after timet=0 has been established for a new cycle, the bandpass filter 364 isapplied to the waveforms. The filter 364 is matched to the resonantfrequency or frequencies of the transmitter and receiver. The bandwidthof the filter should be matched to about the full width at half maximumof the spectral resonances. Generally, the narrower the bandpass filter,the more noise is excluded from the signal and the higher the S/N. Butthe bandpass filter should not be so narrow that it excludes asignificant part of the in-band resonant energy, or that it reducesprobability of detection.

At block 412 a “difference” routine is performed at which the filteredwaveform from each receiver is compared to and subtracted from a runningaverage stored in the second memory 366 of the past n filtered waveformsfrom each respective receiver. The newest filtered waveform isdestructively interfered with the average filtered waveform.

The output of that waveform interference process, the differencewaveform, should only differ substantially from noise for reflectionsfrom targets that are not motionless. At block 414 the differencewaveform is amplified and automatic gain control (AGC) is preferablyprovided. With some changes to the tracking algorithms, differencewaveforms can be created from more than two waveforms, with someimprovement in resolution, particularly cross-range resolution.

At block 416, the calculated data is provided to the processingalgorithms 390, which may be detection algorithms or trackingalgorithms. Different algorithms may be selected for different purposes.The processed signal may be used, for example, for detection of humanswithin a cargo container or for continuous, real-time tracking of humansbehind a wall.

FIG. 18, consisting of FIGS. 18A and 18B, is a waveform chartillustrating operation in a through-wall monitoring system embodiment ofthe transducer apparatus 1. In an example of signal processing for anacoustic through-wall detection system, acoustic pulses are produced andcoupled into a wall 72 for about 15 to 30 seconds at a constant pulserepetition frequency (pro of about 3 per second. This rate allowsreverberations in a 40-ft cargo container 70 to subside significantlybetween pulses. Next, the last n received echo signals are averaged toproduce a baseline signal against which the latest signal is compared.The signal-to-noise ratio (S/N) of the averaged signal is improved byabout n^(1/2). For example, if the last 16 echo signals are averaged,S/N is improved by about 6 dB, or a factor of 4. The latest signal,after being filtered by the bandpass filter 364 at the resonantfrequency, is subtracted from the average in the second memory 366 ofthe last n filtered signals. A difference waveform in excess of apreselected threshold indicates a movement of something on the otherside of the wall 72. The sensor system located outside the wall 72 canbe more sensitive to motion on the same side of the wall 72 than it isto motion on the opposite side of the wall 72 inside the container. Forthat reason, the receiver could be shielded from ambient noise and/orthe system could be operated remotely with no humans nearby, asillustrated in the embodiment in FIG. 2.

The waveforms in FIG. 18 illustrate a comparison of three successivethrough-wall tests, with the acoustic pulses transmitted through amassive desk and a solid hardwood wall. The operator was seated at theposition of the receiver, 6 meters away from the transmitter. Thereceived waveforms were each averaged over 16 pulses. Voltage waveformsused include a first average baseline waveform created with the operatormotionless. With the operator position unchanged, a second averagedwaveform was created. This waveform is overlaid on the first voltagewaveform in FIG. 18A. The second voltage waveform is illustrated ingray. Portions of the first waveform which are not covered by the secondwaveform are illustrated as dark extensions at vertical extremities ofthe second waveform. A third averaged waveform was created as theoperator moved about, and was overlaid on the first average baselinewaveform as the grey waveform in FIG. 18B. The bold waveform in FIG. 18Arepresents the difference between the first and second waveforms. Thebold waveform in FIG. 18B represents the difference between the firstand third waveforms. The large amplitude of the bold waveform in FIG.18B, following a quiescent sound transit time through air, is indicativeof the operator moving about.

FIG. 19, consisting of FIGS. 19A and 19B, illustrates the effects ofapplying a bandpass filter in processing through-wall waveforms. FIGS.19A and 19B compare the same combinations of waveforms in FIGS. 18A and18B, respectively, but with a software bandpass filter about theresonant frequency of the receiving plate. In FIGS. 19A and 19B, thebandpass filter was applied in post-processing to the frequency bandfrom 2101 Hz to 2223 Hz, which spans a filter bandwidth that is 5.6% ofthe central frequency of 2162 Hz. As in FIGS. 18A and 18B, portions ofthe first waveform which are not covered by the second waveform in FIG.19A or by the third waveform in FIG. 19B are illustrated as darkextensions at vertical extremities. The bold waveform in FIG. 19Arepresents the difference between the first and second filteredwaveforms. The bold waveform in FIG. 19B represents the differencebetween the first and third filtered waveforms. The large amplitude ofthe bold waveforms in FIG. 19B, following a quiescent sound transit timethrough air, is indicative of the operator moving about.

A narrowband filter/amplifier is used to create the filtered bandsdescribed above to exclude noise outside the bandpass filter. If thebandpass filter on the receiver is properly matched to the frequency andbandwidth of the transmitted acoustic waves, much of the useful signalwill pass through the filter, thereby increasing S/N. In this example, aproperly designed filter/amplifier could eliminate on the order of about19 dB to 26 dB of noise from the filtered signal, improving the S/N bylike amounts over the embodiment that produced the waveforms in FIG. 18.

The sampling rate requirements of the processor depend upon theoperating frequency of the transducer. In order to accommodateround-trip travel times of acoustic waves in cargo containers that maybe as long as 40 ft, especially when the containers might be clutteredand the acoustic waves might take some time to reverberate andaccumulate at the receiver, the record length is preferably at leastabout 80 ms and need not be more than about 160 ms. According to theNyquist criterion, the lowest sampling rate that could possibly measurethe resonance is about twice the operating frequency, but such a lowrate would lead to aliasing errors. In selected preferred embodiments, asampling rate of five to ten times the operating frequency is suitable.

One suitable preferred pulse repetition frequency (pro of a through-wallsystem is about 3 Hz. This rate is sufficiently high for rapid detectionof motion through a wall, yet allows sufficient time for decay ofreverberations from a previous pulse.

In one preferred embodiment, the t=0 mark on the waveform is establishedby the methods described above. The voltage waveform is centered aboutthe mean voltage. The voltage waveform is stored in memory. A runningaverage of the past n waveforms, where n is 10 to 16, is updated inmemory. A new (i+1)^(th) waveform is subtracted from the running averageof the past n waveforms, up to and including the i^(th) waveform, inorder to produce a difference waveform. A difference waveform exceedinga selected threshold voltage over multiple contiguous samples indicatesa detection.

Every time the new (i+1)^(th) waveform is added to the average, the(i+1−n)^(th) waveform is removed from the running average of the past nwaveforms to update the running average. At a prf of 3 Hz, the firstreal-time detection can be made within about 4 to 5 seconds.

FIG. 20, consisting of FIGS. 20A and 20B, illustrates a method ofreal-time detection by exceedances of a threshold voltage. The data fromthe baseline, second, and third waveforms, shown in FIG. 18 were usedwith a thresholding algorithm that provides an output indicative ofvoltage exceedances above 0.5 V. Since the noise amplitude is almostentirely below 0.5 V, voltage differences above this threshold caused byhuman motion are readily apparent in FIG. 20B.

With the thresholding algorithm illustrated in FIG. 20, detection of amoving person is accomplished without a narrow bandpass filter or fastFourier transforms (FFTs) or inverse FFTs. The detection is achieved byperforming a subtraction of waveforms and amplification of thedifference waveform without even an AGC. Consequently, robustperformance is provided with simplified processing.

With a tracking and locating 3-step algorithm 600, illustrated in FIG.21, a TWS x-y display 680 may be produced that shows the locations andthat tracks in near-real-time all persons on the other side of a wall72. When implemented properly, the algorithm 600 transforms binauralvoltage waveforms from two receivers to the map of current locations ofpersons 680. The algorithm 600 is fast because it emulates the way thehuman mind and eye perceive complex waveforms of thousands of datapoints. Just as the eye quickly picks out periods of thresholdexceedances in voltage waveforms, the algorithm 600 does the same.

The three steps of the tracking and locating algorithm 600 are: (i)Filter noise; (ii) count data points in time bins; and (iii)triangulate. After voltage waveforms from each of two independentreceivers have each been processed through the AGC 380 according to themethod illustrated in FIG. 16, each waveform is subjected to thealgorithm 600 process

In one illustrative example, the waveforms from the AGC 380 representtypical return signals with a 50-ms record length beginning 4 ms afterreceiver triggering and a 100-kHz sampling rate. There are 5000 datapoints in each channel. With this record length and a 4-ms gate, therange of each receiver is 2.3 ft to 30.5 ft. With this range, stereoreceivers separated by 5 ft at the center of one wall are capable oftriangulating range to a target anywhere in a 25-ft×25-ft room (exceptfor the immediate proximity of the receivers).

The first step of the algorithm is to filter out noise by means of avoltage threshold filter 620 applied to each of the two voltagewaveforms from the two data channels. Any data point with a value lessthan an adjustable voltage threshold V₀ is set equal to zero. On adevice in use, the adjustable threshold V₀ should be tunable with abrightness dial 630. The second step of the algorithm is to count howmany data points remain in each time bin after filtering to produce ahistogram 640 of exceedances of the threshold voltage V₀. The recordlength is divided into a number of equal time bins for this purpose. Ifa resolution of 0.5 ft×0.5 ft is desired in the x-y display 680, then wehave found that time bins of 0.5 ms provide adequate resolution, buttime bins of 1 ms do not. If the time bins are too big, then the x-ydisplay 680 may overlook people and show ghost images.

The data in the threshold-exceedance histogram 640 were filtered with abrightness threshold of V₀=3.2. Then the data points remaining afterfiltering in each of 100 0.5-ms bins were counted to produce thehistogram 640.

The third step of the algorithm 600 is triangulation 660, which involvestransforming from range coordinates to x-y coordinates and correlatingthe filtered data in each x-y cell. The triangulation 660 is the mostprocessing-intensive step, but it involves working with time bins,rather than voltage samples, and there are 10,000 times fewer time binsthan voltage samples in this example.

To implement triangulation 660, an x-y grid is established, with plabeling the cells in the x direction, and q labeling the cells in the ydirection. The value assigned to each cell (p,q) is the product of thecount from the ‘appropriate’ time bin of one receiver with the countfrom the ‘appropriate’ time bin of the other receiver.

Suppose P0_(m) is the count of data points from the histogram 640 in them^(th) time bin of one receiver, and P1_(n) is the count in the n^(th)time bin of the other receiver. Suppose that the time bins are 0.5 ms,the x-y cells are 0.5 ft×0.5 ft, the left receiver is 10 ft (20 cells)from the left corner of the room (where p=0), and the right receiver is15 ft (30 cells) from the left corner of the room. Then thecorrelated-count value assigned to each cell (p,q) is

C _(p,q)=(P0_(m))(P1_(n)),

where the ‘appropriate’ time bins are

m=round{K[(p−20)² +q ²]^(1/2)},

n=round{0.5K([(p−20)² +q ²]^(1/2)+[(p−20−2d)² +q ²]^(1/2))}.

Here, “round” is a function that rounds off numbers to the nearestinteger, and K≡2/c=1.77 is a constant that accounts for the doubleround-trip time at the speed of sound in air, c=1.13 ft/ms.

After triangulation 660 was performed on the data from the histogram640, the correlated-count value, C_(p,q), may be displayed as the x-ymap 680 of persons being tracked through the wall 72. The map 680 may bedisplayed as a 3-dimensional surface plot, as in FIG. 21.

The 3-step algorithm 600 is fast, because it emulates human perceptionin picking out only clusters of data points that stand out above thenoise. Then, instead of correlating all of the data points in eachchannel (25 million correlations in this example), the algorithm 600performs only one correlation per x-y grid cell (25 hundred correlationsin this example).

The accuracy of the algorithm 600 in locating persons is limited by thepulse detection error, which is of the order of the return pulse width.The algorithm 600 reduces the numerical resolution to this low level ofaccuracy (a 0.5-ms time bin in this example) before correlating the twochannels. Thus, the algorithm 600 does not need to degrade therealizable locating accuracy. In this example, the tracking resolutionis better than 1 ft at ranges comparable to the receiver separation (5ft in this example), and the cross-range resolution is degraded atranges much longer than this separation.

The algorithm is robust at detecting signals, even at low S/N, and ateliminating false alarms (ghost images), as long as the time bins arechosen small enough (0.5 ms in this example). In this example, two ofthe targets had S/N≈4, yet were easily detected and located by thealgorithm 600. The algorithm 600 is even robust at discriminatingclosely spaced persons.

The algorithm will be easy to use in a TWS device. The only parameter tobe varied by the user is brightness (filter threshold V₀), which may becontrolled by the brightness dial 630, with immediate visual feedback tothe user.

FIG. 22 is an illustration of one embodiment of an FASN unit 500 that isdesigned to operate at variable depths and water pressures. The FASNunit 500 may be anchored in place by a low-cost weight 36, like aconcrete block, and by the tether 18. The sonar head 28 will besuspended at either a fixed or a variable depth below the floating buoy24, as in FIG. 1. A fixed depth for all FASN units in a network ensuresthat the water pressure on the thin plate of each impact transducer isroughly constant and the same throughout the FASN network. Then the airor gas pressure behind the plate can be set during manufacture toequalize the water pressure, if necessary.

A cable from the sonar head 28 to the buoy 24 could transmit telemetrydata to the antenna 26, which could be mounted on the buoy 24. The samecable could transmit control signals from the antenna 26 to the sonarhead 28. The tension reel 32 could remove slack from the tether 18 inchanging tides and waves, and could provide a restoring force to keepthe buoy 24 positioned nearly vertically above the anchor 36 at alltimes.

In the embodiment of a variable-depth sonar head 500 in FIG. 22, thereis a pressurized cavity 550 behind each resonant plate 506. The gaspressure in the cavity 550 can be equilibrated to the external waterpressure by means of a pressure gauge and a regulator at a valve 551 toa compressed-gas tank 554. An excess of gas pressure in the cavity 550can be relieved through the outlet valve 552.

A long-lifetime, autonomous sonobuoy 20 offers low-cost, efficient,compact, and lightweight approaches to the following basic features: i)energy storage/DC power supply; ii) impact transducer; and iii) awatertight housing in a body with flotation.

The sonar head 500 could in general comprise:

i) A watertight pressurized cavity 550

ii) One or more thin transmitting/receiving plates 506

iii) One or more drivers 510 for the plate or plates 506

iv) At least one piezoelectric film sensor 514 for each plate 506

v) A self-contained battery power supply 578

vi) A receiver 520

vii) A DSP board and other signal processing and data storage components530

viii) A data transmitter 534 for transmitting data from the sonar head500

The operating frequency of the impact transducer is chosen mainly as atrade-off between power production and ambient noise. The two principaltypes of ambient noise at inland rivers and waterways, ports andharbors, and near coastal regions are shipping noise and wind noise.Noise is much higher at low frequencies than high frequencies. Inhigh-noise locations and shallow waters, like the Straits of Malacca,the level of ambient noise from shipping could be higher by 30 dB ormore at 1 kHz than at 20 kHz. Nevertheless, it is easier to produceacoustic power at lower frequencies because the transducer is bigger andheavier. In the present illustration, 1 kHz is chosen as the nominaloperating frequency.

For the present point design embodiment, corresponding to Table I, abattery energy/pulse of 3.3 J was needed for an impact transducer toradiate a 1-kHz, 4-ping-ms pulse at an average power of 50 W. A singlehigh-capacity lithium thionyl chloride battery, such as the Saft LS33600 3.6-V “D-cell” size battery, has a 17-A·hr capacity. This capacitycorresponds to 220 kJ of stored energy per battery, enough to produce67,000 pulses, for a 270 ping-second lifetime at 50 W average power. Ata prf of one pulse per 5 minutes, one battery could drive the impacttransmitter of the point design for nearly 8 months, and 8 such “D-cell”size batteries, for 5 years.

An alternative power supply for the point-design sonobuoy 20 comprisesone or two rechargeable batteries and a self-recharging generator. Asthe tension reel 32 attached to the tether 18 (FIG. 1) unwinds andrewinds with wave motion, the spinning reel drives an electricalgenerator, which recharges the batteries. Through a ratchet-like cabledrive, the wave motion acting on the buoy 24 can be made to do work onthe generator, and to produce and store electrical energy in therechargeable batteries, whether the tension reel is unwinding orrewinding. The tether cable 18 should be made of a strong, lightweightmaterial that does not stretch significantly under tension and that hasa specific gravity less than or about equal to that of sea water, 1.03.That will keep the requirements for tension in the reel and buoyancy inthe buoy modest, even in deep littoral waters. Unlike intermittent andunreliable solar power, wave power is virtually continuous. The minimalaverage-power requirements of each point-design sonobuoy 20, includingtransmit/receive functions, 2-way communications, and station-keeping,can easily be provided by a self-recharging generator such as this one,based on wave motion.

The sonar projector can be designed to produce two or morecounter-directed pulses, one each from plates at two opposite faces ofthe sonar head. As long as the wavelength is much greater than the platediameter, each plate acts essentially as a simple source that producesnearly isotropic acoustic radiation.

A dual-plate impact-transmitter design, like that in FIG. 6, has afurther advantage of allowing identically manufactured sonobuoy units toproduce unique pulse signatures, even though they operate at identicalfrequencies, bandwidths, and pulse widths. Each plate of an impacttransmitter has a sharply defined pulse start time t=0. Piezoelectricfilm sensors 137 affixed to each transmitting plate 120 register asharply defined voltage spike at the moment of impact of the impactor onthe plate. The jitter Δt in determining t=0 is much smaller than a waveperiod, and is generally about equal to the voltage sampling time. Thehigh precision to which t=0 is known is used to assign a unique pulsesignature to each dual-plate sonar head 500 after it has beenmanufactured, as explained with respect to FIG. 6.

Upon producing a double pulse, a FASN will transmit to either a relaybuoy or to a central station a signal conveying its location and thetime difference between the start of its two pulses. Any receiver in thesonobuoy network that receives this double pulse and then a short timelater receives the reflected double pulse from a target, such as asubmarine, will transmit a signal conveying its own location and thetimes of receiving the pairs of double pulses. From this information,the range of the target to the receiver is known. If at least two FASNsin the network report a detection, then the position of the target canbe determined by triangulation.

This method of identifying the source depends upon pulses from at leastone FASN reflecting from a target and being received by at least oneother FASN in the network. That means the impact transmitters arepreferably simple sources with nearly isotropic radiation patterns fromeach plate.

In a FASN network, the primary means of ensuring thorough surveillancecoverage of a protected area is by proliferating nodes to achieve thedesired margin of safety.

The active-sonar equation for the monostatic case, in which thetransmitter and receiver are collocated, is given by:

TL=½(SL+TS+DI−NL−DT),

where TL is one-way transmission loss, SL is transmitter source level,TS is target strength, DI is receiving directivity index, NL is level ofambient noise and self-noise, and DT is the detection threshold of thereceiver. Transmitting directivity index, or gain, is included in SL.Since the sonar equation is used here to estimate the maximum range fordetection by a FASN in noisy environments, the range may be safelyassumed to be long enough to be noise-limited rather than reverberationlimited. Then the background level NL is isotropic noise, rather thanreverberation.

Using this active-sonar equation for the monostatic case gives aconservative estimate of the range, because in a network of FASNs, theecho from the target may be received by a FASN that is closer than thetransmitter, in which case the total transmission loss will be less thanthe 2TL used above.

One illustrative embodiment, based on the point design in Table I, hasthe following characteristics: Average pulse power P is 50 W; pulseduration T is 4 ms; transmitting (and receiving) directivity index orgain G is 0 dB; probability of false alarm P_(FA) is 1%; probability ofdetection P_(D) is 50%; and frequency is 1 kHz. The false alarm anddetection requirements for each individual FASN are modest, because theprobability of detection by a network of hundreds of FASNs will be muchcloser to 100%, and the network probability of false alarm will be muchlower than 1%.

The echo pulse will be much longer than the 4-ms transmitter pulse,reducing the effective source level of the transmitter. The typical echoduration produced by a submarine target with multipath propagation inshallow water is 100 ms. The effective source level is therefore reducedby about 10 log (4 ms)/(100 ms)=−14 dB. But because this example is fora double-pulse, dual-plate sonar, with effectively twice the power ortwice the pulse duration, the effective source level is increased by ±3dB. This estimate is conservative, since surveillance by a network offixed sonobuoys will generally allow targets approaching coastal watersto be viewed from a wide range of aspect angles. Accounting for pulsestretching, reverberations in shallow water, and moderate sea states,the sonar range to a submarine at intermediate aspect in average harbornoise of each FASN unit in this example is about 0.25 nautical miles.

The total mass of the embodiment of a dual-plate, 1-kHz sonar head inthis example is less than or about 10 kg. Most of that mass (>5.4 kg) isin the two aluminum plates and the steel impactor. The entire sonar headcan be packaged in a volume of about 1 cu. ft.

A 10-kHz sonar head, on the other hand, could be built much smaller andlighter, primarily because the impact-transducer plates are much smallerand the impactor is much lighter at 10 kHz. The trade-off, however, isthat the impact transmitter produces less power at 10 kHz. In averageharbor noise, however, the range estimate for a 1-kHz sonar at 50-Waverage power is comparable to that of a 10-kHz sonar at 1-W averagepower, even with greater absorption at 10 kHz. The reason is primarilythat average harbor noise is about 17 dB quieter at 10 kHz than at 1kHz. In littoral areas where ambient noise is not as significant, a 50-W1-kHz transmitter would have much greater range than a 1-W 10-kHztransmitter.

Not only is a 10-kHz impact-transducer sonar head much lighter and morecompact, it is also less costly than a 1-kHz sonar head which might costless than or about $1000 in a nominal embodiment. The power requirementsof the 10-kHz system are lower and, to the extent its range may be morelimited, shortcomings in range can be compensated by greaterproliferation of FASNs in the network. As a general rule for mechanicaltransducers used in sonar, however, noisy marine environments favorhigher frequencies and quiet environments favor lower frequencies.

In the foregoing detailed description the present subject matter hasbeen described with reference to specific exemplary embodiments thereof.It will, however, be evident that various modifications and changes maybe made thereto without departing from the broader spirit and scope ofthe present subject matter. The present specification and figures areaccordingly to be regarded as illustrative rather than restrictive. Thespecification is not intended to be exhaustive or to limit the presentsubject matter to the precise forms disclosed.

1. A mechanical transducer comprising a mechanical transmitter fortransducing a mechanical impulse to acoustic energy in dense mediacomprising: an actuator or linear induction motor, a thin plate fortransducing a mechanical impulse to acoustic energy, an impactor ordrive rod comprising a mass, said actuator or linear induction motorcausing said impactor or drive rod to strike or push said platerepetitively and induce oscillations in said plate, said transducercomprising an interface coupling oscillations from said plate to aliquid or solid medium, said actuator or linear induction motorcomprising a source storing energy and a trigger to release storedenergy as mechanical energy to cause said impactor or drive rod tostrike or push said plate.
 2. A mechanical transducer according to claim1 wherein said plate is constructed to vibrate in one or more resonantmodes corresponding to a selected position of impact or pushing by adrive rod on the plate and wherein said impactor or drive rod is mountedto impact or push the plate at the selected position.
 3. A mechanicaltransducer according to claim 2 wherein said plate has shape,dimensions, material, and mounting such as to produce desiredcombinations of levels of acoustic radiation at desired frequencies. 4.A mechanical transducer according to claim 1 wherein said source storingenergy comprises a spring and further comprising a driving mechanism forcompressing the spring, said driving mechanism comprising means forcompressing the spring over a preselected time period, the time periodbeing selected to correspond to a level of power needed to compress thespring.
 5. A mechanical transducer according to claim 1 wherein saidtrigger causing the release of stored energy from said source comprisesa latch.
 6. A mechanical transducer according to claim 1 wherein saidplate is spaced with respect to the said impactor such that said plateis beyond an equilibrium position of said impactor, so that saidimpactor strikes said plate before decelerating significantly againstthe spring's pull, and does not remain in contact with said plate for anappreciable time after striking it.
 7. A mechanical transducer accordingto claim 1 wherein said plate is provided with coupling means to coupleacoustic waves efficiently to a liquid or solid medium.
 8. A mechanicaltransducer according to claim 1 wherein the acoustic coupler comprisesspacers between said plate and a high density solid medium, said spacersbeing proportioned to displace said plate with respect to a solid mediumto use said solid medium as a sound board, and said spacers notinterfering with the oscillations of said plate.
 9. A mechanicaltransducer according to claim 1 wherein said actuator is operated at apreselected repetition rate in which the dormancy between pulses on eachtransmitter in a network is selectable.
 10. A mechanical transduceraccording to claim 1 comprising a combination of transmitting plateswherein said impactor or a combination of impactors is mounted to impactsaid combination of plates in a preselected sequence and at preselectedintervals.
 11. A mechanical transducer according to claim 10 whereindifferences in times between striking plates in said combination ofplates defines a signature.
 12. A mechanical transducer according toclaim 11 wherein plates in said combination of plates have selectablerelative positions to define a selected signature.
 13. A mechanicaltransducer according to claim 1 wherein at least one plate in saidcombination of plates is slidably mounted with respect to said impactoror said drive rod into a selected position at which the location ofimpact by the impactor or pushing by the drive rod corresponds to adesired resonant mode.
 14. A mechanical acoustic transducer according toclaim 1 wherein the transmitting plate is supported to the body of thetransmitter by a rigid frame and acoustic isolators.
 15. A mechanicalacoustic transducer according to claim 1 further comprising a sensor forproducing an electrical signal indicative of striking of a plate by theimpactor.
 16. A mechanical acoustic transducer according to claim 1further comprising at least one resonant receiver having a receiverplate matched to a transmitter plate.
 17. A mechanical acoustictransducer comprising a tunable transmitter, the tunable transmittercomprising: a plate caused to vibrate, the plate having anarea-to-thickness ratio to permit efficient transmission of acousticradiation into a dense medium at the desired frequency, a wire undertension, a supporting structure supporting a perimeter of the plate, thewire under tension being secured to the plate and holding the plate incontact with the supporting structure, a mechanical means of pluckingthe wire to excite oscillations of the plate.
 18. A mechanical acoustictransducer according to claim 17 wherein the plate is a thin, circularaluminum plate, the tensioned wire is substantially perpendicular to theplate, the tensioned wire being attached to the center of the plate, andmechanical means for plucking the tensioned wire, the means for pluckingcausing the plate to oscillate longitudinally at twice the frequency oftransverse wave oscillations on the wire.
 19. A mechanical acoustictransducer according to claim 18 wherein the mechanical means ofplucking the wire is a rotating sawtooth wheel.
 20. A sonobuoycomprising a sonar head, the sonar head comprising: energy storage, atleast one mechanical transducer, watertight body with flotation,watertight pressurized cavity, at least one thin plate, at least oneactuator or linear induction motor and impactor or drive rod for eachplate or combination of plates, at least one piezo-film sensor for eachplate, a self-contained battery power supply, a signal processorincluding memory, and a cable or wireless means of transmitting dataoutside the sonar head.
 21. A sonar head according to claim 20connectable to a buoy for flotation and connectable to an anchor forfixing position by tethers, said tethers to include a tension reel tomaintain said sonar head at a fixed depth below said buoy or at a fixedheight above said anchor, and wherein internal gas pressure on said thinplate of each mechanical transducer is equilibrated to water pressureoutside said sonar head, and wherein batteries, processing, andtelemetry subsystems are protected within said sonar head, said sonarhead being connectable to a communications antenna.
 22. A tension reelaccording to claim 21 connectable to a buoy above and an anchor below bya tether or tethers, said tension reel to respond to wave motion andwater currents acting on said buoy by continuously rotating in onedirection to play out, and in the other direction to retract, saidtether or tethers, the rotation in either or both directions driving anelectrical generator, which charges or recharges electricalenergy-storage devices, such as batteries.
 23. An acoustic transduceraccording to claim 1 comprising a through-wall detection system.
 24. Amethod for processing signals received by an acoustic transducercomprising producing: a filtered waveform in response to a receivedacoustic signal; storing the waveform in a memory; calculating a runningaverage of a preselected number of waveforms most recently received;storing a current updated value of the running average; subtracting anew currently received waveform from the current running average;producing a difference waveform; comparing the difference waveform to athreshold voltage and generating an exceedance for every voltage sampleof the difference waveform in excess of the threshold voltage, andcomparing the incidence of exceedances with a preselected thresholdlevel of incidences indicative of a detection of moving persons orobjects.
 25. A method for processing signals received in an acoustictransducer according to claim 24 further comprising the steps of: priorto producing the filtered waveform, sensing occurrence of a transmitterpulse, producing a trigger signal in response to the transmitter pulse,the trigger signal having a preselected duration, and inhibiting inputsof the received acoustic signal during the preselected duration.
 26. Amethod for processing signals received in an acoustic transduceraccording to claim 24 further comprising the steps of: triggering areceiver signal indicative of the received acoustic signal after a fixeddelay; digitizing voltage waveforms; correlating t=0 of a currentwaveform to an average t=0 for a most recent n waveforms; and amplifierapplied with automatic gain control (AGC).
 27. A method for processingsignals received in an acoustic transducer according to claim 24 furthercomprising the steps of: after producing the difference waveforms fromtwo or more receivers, applying an adjustable threshold filter ofvoltage exceedances for each waveform to produce histograms of saidvoltage exceedances versus range or time of flight for each waveform,and correlating said histograms of said voltage exceedances totriangulate the data from each receiver channel to produce a display oflocations of persons and moving objects on the other side of a wall. 28.A device, comprising: a mechanical acoustic transducer comprising ametal plate; and an induction motor, wherein the induction motorfacilitates excitation of the metal plate to produce an acoustic signal.29. The device of claim 28, wherein the metal plate comprises nickel,copper or aluminum.
 30. The device of claim 28, wherein the metal platecomprises aluminum.
 31. A system comprising a sonar transmitter and asonar receiver, the sonar transmitter being mounted in a housing,wherein the sonar transmitter is isolated from vibrations of thehousing.
 32. The system of claim 31, wherein the housing is watertight.33. The system of claim 32, wherein a water pressure on the housing isequal to a pressure inside the housing.